I know many people who are doing very well in the mobile development freelancing space, at least in the NYC area. Many gigs are still onsite or at least benefit from some kind of physical presence, so if that's a factor, then it's a much more level playing field.

I'm a full-time mobile developer as well. Haven't taken a crack at contracting yet myself, but I think that the key to success is a tried and true one: connections, connections, connections. If you're going on these freelance-farm type job sites, you're in a race to the bottom for poor quality gigs.

Also, I've heard that it's pretty hard to 'get started on the side' because many decent contracts want more of your time than you can reasonably offer when you have a full-time gig. Kind of a chicken-or-the-egg problem with that, but I figure that's the risk of it.